Barrier layer for copper interconnect

ABSTRACT

A device and a method of forming the device is provided. The device includes a dielectric layer overlying a substrate, a conductive line with a sidewall in the dielectric layer, a Ta layer adjoining the sidewall of the conductive line, and a metal oxide formed between the Ta layer and the dielectric layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of co-pending U.S.application Ser. No. 14/204,347, filed on Mar. 11, 2014, which is adivisional of U.S. patent application Ser. No. 13/666,792 (now U.S. Pat.No. 8,722,531), filed on Nov. 1, 2012, entitled “Barrier Layer forCopper Interconnect,” which application is hereby incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to semiconductor devices, and particularlyto copper interconnects and methods for their fabrication.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications,such as personal computers, cell phones, digital cameras, and otherelectronic equipment, as examples. As technology has progressed, thedemand for smaller semiconductor devices with improved performance hasincreased. As feature densities increase, the widths of the conductivelines, and the spacing between the conductive lines of back-end of line(BEOL) interconnect structures also need to scale smaller.

A move is being made away from the traditional materials used in thepast in semiconductor device designs, in order to meet these demands. Toreduce the RC time delay, low dielectric constant (low-k) materials arebeing used as insulating materials, and there is a switch being made tothe use of copper for interconnect materials, rather than aluminum.Advantages of using copper for semiconductor device interconnectsinclude abilities to operate faster and manufacture thinner conductivelines because copper has lower resistivity and increasedelectromigration resistance compared to aluminum. Combining copperinterconnects with low-k dielectric materials increases interconnectspeed by reducing the RC time delay, for example.

Copper interconnects are often formed using damascene processes ratherthan by direct etching. Damascene processes are typically either singleor dual damascene, which includes forming openings by patterning andetching inter-metal dielectric (IMD) layers and filling the openingswith copper. Because copper diffuses easily into some dielectricmaterials, especially some types of low-k dielectric materials, adiffusion barrier layer is usually deposited on the inner walls of thedamascene opening before the copper is formed. Refractory metals such astantalum (Ta) or titanium (Ti), or nitride compounds of these metals areused as materials of the diffusion barrier layer. However, there aresome challenges in using refractory metals in the copper damascenestructure since these metallic films have high resistance, therebycausing increased resistance in the copper lines and increased RC delay,especially in small, narrow features.

As the shrinkage of copper lines has progressed in recent years, thereis a trend towards thinner films being used for the diffusion barrierlayer. Physical vapor deposition (PVD) process used for depositing athinner TaN/Ta barrier layer encounters difficulties in advanced scaleof interconnection. Atomic layer deposition (ALD) process is thecandidate to deposit a very thin diffusion barrier layer with uniformcoverage, but the ALD method is disadvantageous in extremely lowdeposition rate and poor throughput. In addition, in manufacturing theTaN/Ta film, a problem occurs in which favorable adhesion betweendiffusion barrier layer and the IMD layer cannot be achieved. Forexample, copper lines peel off at the interface, worsening the yield ofthe semiconductor device.

Therefore, there is a need for an improved diffusion barrier layer inthe copper interconnect, and a method of forming thereof.

BRIEF DESCRIPTION OF THE DRAWING

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion

FIG. 1 is a flowchart illustrating a method for fabricating asemiconductor device comprising an interconnect according to variousaspects of the present disclosure; and

FIGS. 2-8 show schematic cross-sectional views of an interconnectstructure of a semiconductor device at various stages of fabricationaccording to various aspects of the present disclosure.

DETAILED DESCRIPTION

Embodiments provide a barrier layer formed in a copper interconnectstructure of a semiconductor device and methods of forming thereof,which has wide applicability to many manufacturers, factories andindustries, including integrated circuit fabrications, microelectronicfabrications, and optical electronic fabrications. Reference will now bemade in detail to the present embodiments, examples of which areillustrated in the accompanying drawings. Wherever possible, the samereference numbers are used in the drawings and the description to referto the same or like parts. In the drawings, the shape and thickness ofone embodiment may be exaggerated for clarity and convenience. Thisdescription will be directed in particular to elements forming part of,or cooperating more directly with, apparatus in accordance with thepresent invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art. Further, when a layer is referred to as being on another layeror “on” a substrate, it may be directly on the other layer or on thesubstrate, or intervening layers may also be present.

FIG. 1 is a flowchart illustrating a method 100 for fabricating asemiconductor device 200 according to various aspects of the presentdisclosure. FIGS. 2-8 show schematic cross-sectional views of asemiconductor device 200 at various stages of fabrication according toan embodiment of the method 100 of FIG. 1. The semiconductor device 200may be included in a microprocessor, memory cell, and/or otherintegrated circuit (IC). It is noted that the method of FIG. 1 does notproduce a completed semiconductor device 200. A completed semiconductordevice 200 may be fabricated using complementarymetal-oxide-semiconductor (CMOS) technology processing. Accordingly, itis understood that additional processes may be provided before, during,and after the method 100 of FIG. 1, and that some other processes mayonly be briefly described herein. Also, FIGS. 1 through 8 are simplifiedfor a better understanding of the present disclosure. For example,although the figures illustrate the semiconductor device 200, it isunderstood the IC may comprise a number of other devices comprisingtransistors, resistors, capacitors, inductors, fuses, etc.

Referring to FIGS. 1 and 2, the method 100 begins at step 102 whereinrecess cavities 208 are formed in a dielectric layer 206. In someembodiments, the dielectric layer 206 is referred to an inter-metaldielectric (IMD) layer. In some embodiments, a first etch stop layer 204is formed over a semiconductor substrate 202 and under the dielectriclayer 206. In some embodiments, the recess cavities 208 are formed inthe dielectric layer 206 and the first etch stop layer 204. Thesemiconductor substrate 202 is a substrate as employed in asemiconductor integrated circuit fabrication, and integrated circuitsmay be formed therein and/or thereupon. The term “semiconductorsubstrate” is defined to mean any construction comprising semiconductormaterial, for example, a silicon substrate with or without an epitaxiallayer, a silicon-on-insulator substrate containing a buried insulatorlayer, or a substrate with a silicon germanium layer. The term“integrated circuits” as used herein refers to electronic circuitshaving multiple individual circuit elements, such as transistors,diodes, resistors, capacitors, inductors, and other active and passivesemiconductor devices. A conductive region formed in and/or on thesemiconductor substrate 202 is a portion of conductive routes and hasexposed surfaces that may be treated by a planarization process, such aschemical mechanical polishing. Suitable materials for the conductiveregions may include, but not limited to, for example copper, aluminum,copper alloy, or other mobile conductive materials. Copper interconnectlevel may be the first or any subsequent metal interconnect level of thesemiconductor device.

In some embodiments, the first etch stop layer 204 functions forcontrolling the end point during subsequent etching processes. In someembodiments, the first etch stop layer 204 is formed of silicon oxide,silicon nitride, silicon carbide, silicon oxynitride or combinationsthereof. In some embodiments, the first etch stop layer 204 has athickness of about 10 angstroms to about 1000 angstroms. The first etchstop layer 204 is formed through any of a variety of depositiontechniques, including, LPCVD (low-pressure chemical vapor deposition),APCVD (atmospheric-pressure chemical vapor deposition), PECVD(plasma-enhanced chemical vapor deposition), PVD (physical vapordeposition), sputtering, and future-developed deposition procedures.

The dielectric layer 206 may be a single layer or a multi-layeredstructure. In some embodiments, the dielectric layer 206 with athickness varies with the applied technology, for example a thickness ofabout 1000 angstroms to about 30000 angstroms. In some embodiments, thedielectric layer 206 is silicon oxide, carbon-doped silicon oxide, acomparatively low dielectric constant (k value) dielectric material witha k value less than about 4.0, or combinations thereof. In someembodiments, the dielectric layer 206 is formed of a material, includinglow-k dielectric material, extreme low-k dielectric material, porouslow-k dielectric material, and combinations thereof. The term “low-k” isintended to define a dielectric constant of a dielectric material of 3.0or less. The term “extreme low-k (ELK)” means a dielectric constant of2.5 or less, and preferably between 1.9 and 2.5. The term “porous low-k”refers to a dielectric constant of a dielectric material of 2.0 or less,and preferably 1.5 or less. A wide variety of low-k materials may beemployed in accordance with embodiments, for example, spin-on inorganicdielectrics, spin-on organic dielectrics, porous dielectric materials,organic polymer, organic silica glass, FSG (SiOF series material), HSQ(hydrogen silsesquioxane) series material, MSQ (methyl silsesquioxane)series material, or porous organic series material. In some embodiments,the dielectric layer 206 is deposited through any of a variety oftechniques, such as chemical vapor deposition (CVD), physical vapordeposition (PVD), atomic layer deposition (ALD), remote plasma enhancedchemical vapor deposition (RPECVD), liquid source misted chemicaldeposition (LSMCD), coating, spin-coating or another process that isadapted to form a thin film layer over the substrate.

In embodiments, the dielectric layer 206 is a nitrogen-containing layer,a carbon-containing layer, or a carbon-containing andnitrogen-containing layer for increasing corrosion resistance during asubsequent chemical mechanical polishing (CMP) process and/or increasingelectromigration resistance. In one embodiment, the dielectric layer 206is a silicon-containing and nitrogen-containing dielectric layer. Inanother embodiment, the dielectric layer 206 is a silicon-containing andcarbon-containing dielectric layer. In other embodiment, the dielectriclayer 206 is a silicon-containing, nitrogen-containing, andcarbon-containing dielectric layer. In one embodiment, the dielectriclayer 206 has a ratio by weight of carbon to silicon about equal orgreater than 0.5. In another embodiment, the dielectric layer 206 has aratio by weight of nitrogen to silicon about equal or greater than 0.3.In other embodiment, the dielectric layer 206 has a ratio by weight ofcarbon to silicon about equal or greater than 0.5 and a ratio by weightof nitrogen to silicon about equal or greater than 0.3.

The recess cavities 208 are exemplary dual damascene openings includingan upper trench section 208U and a lower via-hole section 208L patternedin the dielectric layer 206 and the first etch stop layer 204 to definea contact region on the semiconductor substrate 202. Although theembodiments illustrate dual damascene openings in the IMD layer, the useof single damascene openings in the IMD layer also provide values. Indual damascene techniques including a “via-first” patterning method or a“trench-first” patterning method, the upper trench section 208U and thelower via-hole section 208L may be formed using a typical lithographicwith masking technologies and anisotropic etch operation (e.g. plasmaetching or reactive ion etching). In alternative embodiments, a bottometch stop layer (not shown), a middle etch stop layer (not shown), apolish stop layer (not shown), or an anti-reflective coating (ARC) layer(not shown) is deposited on or intermediately in the dielectric layer206, providing a clear indicator of when to end a particular etchingprocess.

Referring to FIGS. 1 and 3, the method 100 proceeds to step 104 whereina copper-containing (Cu-containing) layer 210 is formed on theabove-described structure to line the sidewalls and bottoms of therecess cavities 208 and over the dielectric layer 206. In oneembodiment, the Cu-containing layer 210 is a metal alloy layercontaining at least a main metal element, copper (Cu), and an additivemetal element. In some embodiments, the additive metal element includesmanganese (Mn), aluminum (Al), titanium (Ti), niobium (Nb), chromium(Cr), vanadium (V), yttrium (Y), technetium (Tc), rhenium (Re), cobalt(Co), or combinations thereof.

In some embodiments, the Cu-containing layer 210 is copper-manganese(CuMn) layer with a crystal structure of face center cubic (FCC). Insome embodiments, a ratio of manganese to copper contained in the CuMnlayer is ranging from about 0.05% to about 10.00%. In some embodiments,the Cu-containing layer 210 has a thickness of about 1 angstrom to about150 angstroms. In some embodiments, the Cu-containing layer 210 has athickness ranging from about 5 angstroms to about 50 angstroms. In someembodiments, the Cu-containing layer 210 is deposited by using physicalvapor deposition (PVD), chemical vapor deposition (CVD), plasma enhancedCVD (PECVD), low pressure CVD (LPCVD), or other well-known depositiontechniques.

Referring to FIGS. 1 and 4, the method 100 proceeds to step 106 whereina barrier layer 212 is formed over the Cu-containing layer 210. Thebarrier layer 212 may function as a barrier to prevent a subsequentlyformed conductor from diffusing into the underlying dielectric layer206. In some embodiments, the barrier layer 212 includes tantalum (Ta),titanium (Ti)or the like. In some embodiments, the barrier layer 212 hasa thickness of about 10 angstrom to about 250 angstroms. In someembodiments, a combined thickness of the Cu-containing layer 210 and thebarrier layer 212 is less than about 120 angstroms to prevent a gapfilling issue during the subsequent opening filling process. There aretwo different phases for Ta layer, including α phase tantalum (α-Ta)layer with a body center cubic (BCC) phase and β phase tantalum (β-Ta)layer with a tetragonal phase. The α-Ta layer has a resistivity lowerthan a resistivity of the β-Ta layer. It tends to form a α-Ta layer ifthe underlying layer has a crystal structure.

In some embodiments, the barrier layer 212 is α-Ta layer. In someembodiments, the barrier layer 212 has a resistivity less than about 60μΩ/cm. Therefore, a low resistance of the barrier layer 212 can beachieved to decrease a RC delay issue. In alternative embodiments, thebarrier layer 212 is a Ta layer with α phase and/or β phase. In someembodiments, the barrier layer 212 is deposited by using PVD, CVD,PECVD, LPCVD, or other well-known deposition techniques.

Referring to FIGS. 1 and 5, the method 100 proceeds to step 108 whereina conductor 214 is formed over the barrier layer 212 to fill the recesscavities 208. In some embodiments, the conductor 214 is formed as aninterconnect structure in the dielectric layer 206. In some embodiments,the conductor 214 is deposited by an electro-chemical plating (ECP)process. In some embodiments, the conductor 214 at least contains themain metal element, e.g., copper (Cu). In some embodiments, theconductor 214 further contains an additive metal element. In someembodiments, the conductor 214 contains an additive metal elementdifferent from the additive metal element in the Cu-containing layer210. In some embodiments, the additive metal element in the conductor214 is tantalum, indium, tin, zinc, manganese, chromium, titanium,germanium, strontium, platinum, magnesium, aluminum or zirconium.

In some embodiments, a conductive seed layer (not shown) is formedbefore forming the conductor 214. In some embodiments, the conductiveseed layer at least contains the main metal element, e.g., copper (Cu),as contained in the conductor 214. In some embodiments, the conductiveseed layer is a Cu-containing layer, such as CuAl, CuMn, or the like.

Referring to FIGS. 1 and 6, the method 100 proceeds to step 110 whereinthe portion of conductor 214 over the dielectric layer 206 is removed.In some embodiments, the removing process is chemical mechanicalpolishing (CMP) process performed to remove the excess portions of theconductor 214, the conductive seed layer, the barrier layer 212, and theCu-containing layer 210 outside the recess cavities 208, thus exposingthe top surface of the dielectric layer 206 and achieving a planarizedsurface.

Referring to FIGS. 1 and 7, the method 100 proceeds to step 112 whereina second etch stop layer 216 is formed on the above-described planarizedsurface. The second etch stop layer 216 may control the end point duringsubsequent etching processes. In some embodiments, the second etch stoplayer 216 is formed of silicon oxide, silicon nitride, silicon carbide,silicon oxynitride or combinations thereof. In some embodiments, thesecond etch stop layer 216 has a thickness of about 10 angstroms toabout 1000 angstroms. In some embodiments, the second etch stop layer216 is formed through any of a variety of deposition techniques,including LPCVD APCVD, PECVD, PVD, and sputtering.

Referring to FIGS. 1 and 8, the method 100 proceeds to step 114 whereina thermal treatment 218 is performed on the semiconductor substrate 202.In some embodiments, the thermal treatment 218 includes an annealingprocess. In one embodiment, the thermal treatment 218 is performed at atemperature ranged from about 137° C. to about 600° C. In anotherembodiment, the thermal treatment 218 is performed at a temperatureranged from about 280° C. to about 400° C. In one embodiment, thethermal treatment 218 is performed at a duration ranged from about 10min. to about 600 min. In some embodiments, the thermal treatment 218 isperformed using furnace, rapid thermal processing (RTP), or hot plateequipment.

During and/or after the annealing process, in some embodiments, theadditive metal element in the Cu-containing layer 210 partially orcompletely diffuses to the surface of the dielectric layer 206. In someembodiments, the diffused additive metal element reacts with thedielectric layer 206 to form a diffusion barrier layer 220. Thediffusion barrier layer 220 is formed in a self-aligned manner at theboundary between the dielectric layer 206 and the Cu-containing layer210. In one embodiment, the diffusion barrier layer 220 has a thicknessranging from about 1 angstrom to about 30 angstroms. In anotherembodiment, the diffusion barrier layer 220 has a thickness ranging fromabout 1 angstrom to about 15 angstroms. In some embodiments, theformation of the diffusion barrier layer 220 consumes some of thedielectric layer 206 and the Cu-containing layer 210. In one embodiment,the diffusion barrier layer 220 is a metal oxide layer because theoxygen existed in the dielectric layer 206 reacts with the additivemetal element in the Cu-containing layer 210 during the annealingprocess. In some embodiments, the metal oxide layer is MnOx and/orMnSiyOz. In alternative embodiments, the metal oxide layer is TiOx,AlOx, CoOx, VOx, YOx, TcOx, ReOx, or combinations thereof. In someembodiments, the diffusion barrier layer 220 has a weight ratio ofmanganese to oxygen from about 0.1 to about 10. The diffusion barrierlayer 220 may function as a protector to prevent the conductor 214diffusing into the dielectric layer 206.

After the annealing process, the Cu-containing layer 210 is transformedto be a post Cu-containing layer 210′. In one embodiment, the postCu-containing layer 210′ contains the main metal element and theadditive metal element remained therein, wherein the content of theadditive metal element in the post Cu-containing layer 210′ is less thanthe content of the additive metal element in the Cu-containing layer210. In another embodiment, the post Cu-containing layer 210′ containsthe main metal element but without the additive metal element becausethe additive metal element has been consumed entirely after theannealing process.

The step of thermal treatment can be provided by any thermal processingafter forming the Cu-containing layer 210. In alternative embodiments,the step of thermal treatment is provided at the time immediately afterthe formation of the Cu-containing layer 210, immediately after the stepof CMP of removing the excess portions of the conductor 214 outside therecess cavities 208, immediately after the step of forming the secondetch stop layer 216, or after the step of forming a passivation layer ona top metal layer.

A benefit of the dislocations for the semiconductor device and theinterconnect structure, in accordance with the disclosed embodiments, isthat the diffusion barrier layer formed in a self-aligned manner cansolve the contact issue to improve package capabilities. Further, thebarrier layer over the diffusion barrier layer may further prevent theinterconnect conductor from diffusing into the IMD layer to ensure thepackage capabilities. Moreover, the barrier layer includes Ta with αphase can achieve low resistivity to decrease contact resistance. Thesecan improve device performance.

In one embodiment, a device includes a dielectric layer overlying asubstrate, a conductive line with a sidewall in the dielectric layer, aTa layer adjoining the sidewall of the conductive line, and a metaloxide between the Ta layer and the dielectric layer.

In another embodiment, a device includes a dielectric layer overlying asubstrate, an opening in the dielectric layer, a metal-containing layeroverlying the opening, a barrier layer overlying the metal-containinglayer, a conductive layer in the opening, and a metal oxide barrierlayer underlying the conductive layer.

In still another embodiment, a device includes a dielectric layeroverlying a substrate, a recess cavity in the dielectric layer, a CuMnlayer overlying the recess cavity, a α phase tantalum layer overlyingthe CuMn layer within the recess cavity, a conductive layer overlyingthe α phase tantalum layer within the recess cavity, and a MnOx barrierlayer underlying the conductive layer.

In yet still another embodiment, a method of forming a device isprovided. The method includes forming a dielectric layer overlying asubstrate and forming an opening in the dielectric layer. Ametal-containing layer is formed along sidewalls of the opening, and aconductive layer is formed over the metal-containing layer. Thesubstrate is treated causing the metal-containing layer to react withthe dielectric layer to form a diffusion barrier layer.

In yet still another embodiment, a method of forming a device isprovided. The method includes forming a dielectric layer overlying asubstrate and forming an opening in the dielectric layer. Ametal-containing layer is formed along sidewalls of the opening, themetal-containing layer having a main metal element and an additive metalelement, and a conductive layer is formed over the metal-containinglayer. The additive metal element is diffused to the dielectric layeralong sidewalls of the opening, thereby forming a diffusion barrierlayer.

In yet still another embodiment, a method of forming a device isprovided. The method includes forming a dielectric layer overlying asubstrate and forming an opening in the dielectric layer. Ametal-containing layer is formed along sidewalls of the opening, themetal-containing layer having a main metal element and an additive metalelement, and a first conductive layer is formed over themetal-containing layer. A second conductive layer is formed over thefirst conductive layer, the second conductive layer filling the opening.The additive metal element is diffused to the dielectric layer alongsidewalls of the opening, thereby forming a compound with the additivemetal element.

Although the present invention has been described in its preferredembodiments, it is not intended to limit the invention to the preciseembodiments disclosed herein. Those skilled in this technology can stillmake various alterations and modifications without departing from thescope and spirit of this invention. Therefore, the scope of the presentinvention shall be defined and protected by the following claims andtheir equivalents.

What is claimed is:
 1. A method comprising: forming a dielectric layeroverlying a substrate; forming an opening in the dielectric layer;forming a metal-containing layer along sidewalls of the opening; forminga conductive layer over the metal-containing layer; and treating thesubstrate, the treating causing the metal-containing layer to react withthe dielectric layer to form a diffusion barrier layer.
 2. The method ofclaim 1, wherein the diffusion barrier layer and the metal-containinglayer comprises a first metal.
 3. The method of claim 2, wherein thefirst metal comprises manganese (Mn), aluminum (Al), titanium (Ti),niobium (Nb), chromium (Cr), vanadium (V), yttrium (Y), technetium (Tc),rhenium (Re), cobalt (Co), or combinations thereof.
 4. The method ofclaim 2, wherein the metal-containing layer has a lower concentration ofthe first metal than the diffusion barrier layer.
 5. The method of claim1, further comprising forming a barrier layer over the metal-containinglayer, prior to forming the conductive layer, wherein the conductivelayer is formed over the barrier layer.
 6. The method of claim 5,wherein the barrier layer has a crystal structure of body center cubic(BCC) structure.
 7. The method of claim 1, wherein the diffusion barrierlayer has a resistivity less than about 60 μΩ/cm.
 8. The method of claim1, wherein the metal-containing layer has a crystal structure of facecenter cubic (FCC) structure.
 9. A method comprising: forming adielectric layer overlying a substrate; forming an opening in thedielectric layer; forming a metal-containing layer along sidewalls ofthe opening, the metal-containing layer having a main metal element andan additive metal element; forming a conductive layer over themetal-containing layer; and diffusing the additive metal element to thedielectric layer along sidewalls of the opening, thereby forming adiffusion barrier layer.
 10. The method of claim 9, wherein thediffusing is performed prior to the forming the conductive layer. 11.The method of claim 9, wherein, after the diffusing, themetal-containing layer has a lower concentration of the additive metalelement than the diffusion barrier layer.
 12. The method of claim 11,wherein the diffusion barrier layer comprises an oxide layer.
 13. Themethod of claim 12, wherein the oxide layer has a thickness less thanabout 150 angstroms.
 14. The method of claim 9, further comprisingforming a barrier layer between the metal-containing layer and theconductive layer.
 15. The method of claim 9, wherein themetal-containing layer has a thickness from about 5 angstroms to about50 angstroms.
 16. A method comprising: forming a dielectric layeroverlying a substrate; forming an opening in the dielectric layer;forming a metal-containing layer along sidewalls of the opening, themetal-containing layer having a main metal element and an additive metalelement; forming a first conductive layer over the metal-containinglayer; and forming a second conductive layer over the first conductivelayer, the second conductive layer filling the opening; and diffusingthe additive metal element to the dielectric layer along sidewalls ofthe opening, thereby forming a compound with the additive metal element.17. The method of claim 16, wherein the diffusing is performed prior tothe forming the first conductive layer.
 18. The method of claim 16,wherein the diffusing is performed prior to the forming the secondconductive layer and after the forming the first conductive layer. 19.The method of claim 16, further comprising further comprising forminganother dielectric layer over the second conductive layer.
 20. Themethod of claim 19, wherein the diffusing is performed after the formingthe another dielectric layer.